The Request

Ethernet Envelope

ARP begins inside an Ethernet frame, and the frame says this local question is broadcast.

highlighted = computed this step

Start with the local envelope

ARP starts as an Ethernet frame on one local link. The envelope says who should hear the question before the ARP fields begin.

ethertype=0x0806\text{ethertype}=0x0806
Ethernet envelopeThe Ethernet header is decoded from the pinned ARP request bytes.Ethernet envelope112 bits / 14 bytes0xffffffffffff0011223344550806byte 0byte 1byte 2byte 311111111111111111111111111111111byte 4byte 5byte 6byte 711111111111111110000000000010001byte 8byte 9byte 10byte 1100100010001100110100010001010101byte 12byte 130000100000000110dst-mac0xffffffffffff(cont.)(cont.)src-mac0x001122334455ethertype2054->ARP

Broadcast the question

The destination MAC is 0xffffffffffff, the broadcast address. The client MAC is 0x001122334455.

dst=0xffffffffffff,src=0x001122334455\text{dst}=0xffffffffffff,\quad \text{src}=0x001122334455
Ethernet envelopeThe Ethernet header is decoded from the pinned ARP request bytes.Ethernet envelope112 bits / 14 bytes0xffffffffffff0011223344550806byte 0byte 1byte 2byte 311111111111111111111111111111111byte 4byte 5byte 6byte 711111111111111110000000000010001byte 8byte 9byte 10byte 1100100010001100110100010001010101byte 12byte 130000100000000110dst-mac0xffffffffffff(cont.)(cont.)src-mac0x001122334455ethertype2054->ARP

Name the payload

The EtherType field is 0x0806, so the payload is ARP rather than an IP packet.

EtherType=0x0806\text{EtherType}=0x0806
Ethernet envelopeThe Ethernet header is decoded from the pinned ARP request bytes.Ethernet envelope112 bits / 14 bytes0xffffffffffff0011223344550806byte 0byte 1byte 2byte 311111111111111111111111111111111byte 4byte 5byte 6byte 711111111111111110000000000010001byte 8byte 9byte 10byte 1100100010001100110100010001010101byte 12byte 130000100000000110dst-mac0xffffffffffff(cont.)(cont.)src-mac0x001122334455ethertype2054->ARP

Summary

The request is local broadcast first, then ARP payload. The strip checks the destination MAC, source MAC, and EtherType from the exact bytes.

ARP EtherType=0x0806\text{ARP EtherType}=0x0806
Ethernet envelopeThe Ethernet header is decoded from the pinned ARP request bytes.Ethernet envelope112 bits / 14 bytes0xffffffffffff0011223344550806byte 0byte 1byte 2byte 311111111111111111111111111111111byte 4byte 5byte 6byte 711111111111111110000000000010001byte 8byte 9byte 10byte 1100100010001100110100010001010101byte 12byte 130000100000000110dst-mac0xffffffffffff(cont.)(cont.)src-mac0x001122334455ethertype2054->ARP